You may have a faulty capacitor, especially a chip's monolithic bypass capacitor on one of the preamp digital chips. Capacitors are sensitive to temperature, and when a bypass chip goes faulty or dodgy, the result is often a very loud rushing noise.
Another possibility might be a faulty solder connection. As the temperature inside the amp chassis increases, a broken connection will separate. Then, left to cool, the crack will contract and the circuit will function near normal levels. Our shop used to see loads of faulty solder connections, especially when wave soldering became the norm.
Just offering. Good luck!
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